Amar Rabbi Akiva ashreichem Yisrael

Amar Rabbi Akiva ashreichem Yisrael.
Lifnei mi atem mitharin umi metaher etchem?
Avichichem shbashamayim, shene'emar vezarakti aleichem mayim tehorim vetihartem.
V'omer mikveh yisrael Hashem.
Mah mikveh metaher et hatmeyim af hakodesh boruch hu metaher et Yisrael.

Rabbi Akiva said:
You should be happy, Oh Israel. In front of whom do you purify yourselves and who purifies you? Your father in heaven.

As it says, "I will toss pure waters on all of you and you will be purified." (Ezekiel 36:25)

And it says: "HaShem is the hope of Israel." (Jeremiah 17:13) (The word for hope, "mikveh," is identical to the Rabbinic Hebrew word for a ritual bath. However, this term never appears Biblically with this meaning, except for the phrase "mikveh mayyim." see Lev. 11:36, and Exodus 7:19)
Like a mikvah purifies the impure, the Holy One Blessed Be He purifies Israel.

(User-contributed translation.)

We sing it on Lag Ba'Omer.

From the last Mishnah in Mesechet Yoma
Sung after Neila on Yom Kippur.